In the manufacture of microelectronic devices, it has been proposed to deposit solder onto a bond pad by solder-jetting. Solder-jetting comprises projecting a discrete microdroplet from a solder-jet nozzle onto the bond pad, usually composed of copper. As used herein, discrete microdroplet refers to a single microdroplet, typically 25 to 50 .mu.m diameter, that is individually formed and projected onto the bond pad. This differs from techniques, such as thermal spraying, that deposit a plurality of microdroplets concurrently onto the bond pad to fuse the microdroplets into an integral layer. The solder bump is attached to the bond pad and is subsequently reflowed in contact with a lead of an electrical component to bond the component to the substrate and electrically connect the component to the bond pad.
One problem with current solder application techniques is poor bonding between the microdroplet and the faying surface of the bond pad. This is attributed to premature solidification of the microdroplet prior to copper-tin intermetallic formations, which wet the copper surface. This wetting is essential in forming a strong metallurgical bond. This in turn is attributed to cooling of the solder microdroplet during flight and by the bond pad upon impact. The problem is magnified in solder-jetting applications due to the small size of solder microdroplets. As a result, the solder bumps tend to spall off the bond pads prior to making the desired connection to the electrical component. Without proper solder connections at the bond pads, electrical connections between electronic components cannot be made, thereby rendering the product being manufactured inoperable.
Therefore, a need exists for a method for depositing, by solder-jetting or the like, microdroplets onto a copper faying surface to improve adhesion of a solder bump. Also, a need exists to provide a reliable source of solder for forming subsequent electrical and mechanical connections to electronic components mounted onto a substrate.